1. Field of the Invention
The invention relates to apparatus for scanning a disc-shaped record carrier having a track in the form of a spiral which substantially surrounds a point of rotation of the record carrier, said apparatus comprising a record carrier driving device for causing the record carrier to rotate about the point of rotation; a scanning head producing a scanning point on the track; a scanning head support, for example, a slide which is radially displaceable with respect to the record carrier; drive means for radially displacing the scanning head support; and a control system for maintaining the scanning point on the track. Such control system comprises a fine-positioning device arranged on the scanning head support for radially displacing the scanning point across the record carrier within a predetermined limited range of displacement by varying the position of the fine-positioning device with respect to the scanning head support, means for supplying a position-indication signal which is indicative of the position of the fine-positioning device, means for detecting when the position-indication signal exceeds a predetermined threshold value, and control means for temporarily energizing the scanning head support driving device when the threshold value is exceeded so as to displace the scanning head support towards a position at which the position-indication signal resumes a value below the threshold value.
2. Description of the Related Art
Such apparatus is known from GB-B-1,429,882. This document describes a reading device for optical record carriers. In this reading device an optical scanning head is used which produces a focused light beam directed onto the record carrier, a scanning spot being formed on the record carrier at the location of the scanning point. The scanning spot can be radially displaced within a limited range across the record carrier by means of a fine-positioning device in the form of a pivotable mirror. The scanning head includes the pivotable mirror and is mounted on a slide which can be radially displaced by means of the driving device. The driving device comprises a motor-driven spindle which engages a rack provided on the slide.
The radial position of the slide is continuously corrected in dependence upon the position-indication signal so as to maintain the pivotable mirror substantially proximate to the centre of its range of displacement. Furthermore, said document states that it is possible to activate the driving device for the slide only when the excursion of the pivotable mirror has exceeded an extreme position thereof.
In such known apparatus control of the slide position is by a feedback loop. This feedback creates problems when the variable transmission ratio of the spindle/rack combination comprised by the driving device is increased in order to increase the access speed when searching for the position of particular information on the record carrier. Due to hysteresis of the driving system, the displacement of the scanning head will often exhibit an unwanted oscillatory character at large transmission ratios. These oscillations have a detrimental influence on the fine-positioning of the scanning point. Moreover, they result in a higher energy consumption and may also produce an audible production of sound and extra wear.